Easy typed test mocks with Jest and TypeScript

Here’s a quick and simple way to create type-safe test mocks with Jest in TypeScript:

const fooMock: jest.Mocked<IFoo> = {
	fooMethod: jest.fn()
}

const testSubject = new ClientClass(fooMock);

This has a few useful properties:

• The type system will enforce that the dependency is the correct type, i.e. that ClientClass is being passed the instance of IFoo that it requires.
• The type system will enforce that the mock implements the required interface, i.e. that it has stubs for all the required methods.
• The type system can still see that the methods on the mock are jest.fn, allowing things like fooMock.fooMethod.mockReturnValue({}) without complaining.
• The IntelliJ IDE (e.g. WebStorm or IDEA) can automatically fill in the method stubs for you once it sees the jest.Mocked<IFoo> type annotation.

You can also use TypeScript’s handy Pick utility type to get more specific type-safe mocks:

const fooMock: Pick<jest.Mocked<IFoo>, "fooMethod"> = {
	fooMethod: jest.fn()
}

Tech mentioned

• JavaScript
• jest
• nodejs
• TypeScript